Error getting products from the logged in user. ASP.NET Core MVC

I'm having trouble getting only products purchased by the logged in user, that is, the user who made the purchase.

I have a Users table that contains general registration data, and a Customers table.

The relationship between the tables is as follows:

• User: userId, name, email, password
• Customer: customerId, status, section_id, user_id
• Product: productId, name, price, purchase_date, customer_id

Well, the Products table is related to the Customers table and not to the Users table. And I can't change that because there are other applications consulted in the database.

I try to get the products this way, but the id value of Customer is null.

public IActionResult View(int id)
{
    User user = _loginUser.GetUser();
    Product product = _pedidoRepository.GetProduct(id);

    if (product.customer_id != user.Customer.customerId)
    {
        return new ContentResult() { Content = "Access denied" };
    }

    return View(product);
}

I get the error:

Project.Models.users.Customer.get returned null

How can I make the Client id accessible for comparison purposes?

My GetUser method looks like this:

public user GetUser()
{
    // Deserializar
    if( _session.Exists(Key)) 
    {
        string user JSONString = _session.Consult(Key);
        return JsonConvert.DeserializeObject<user>(userJSONString);
    }
    else
    {
        return null;
    }
}

It is important to say that the relationship between "User" and "Customer" is one by one. When registering "User", "Customer" is registered in the table with a fk for "User"

Class User.cs

public class User
{
    /* PK */
    [Key]
    public int userId { get; set; }
    public string name { get; set; }
    public string email{ get; set; }
    public string passwor{ get; set; }

    [ForeignKey("user_id")]
    [JsonIgnore]
    public Customer Customer { get; set; }
}

Class Customer.cs

public class Customer
{
    /* PK */
    [Key]
    public int customerId{ get; set; }
    public string Status { get; set; }
   
    [Column("user_id")]
    [ForeignKey("User")]
    public int? user_id { get; set; }
    public virtual User User { get; set; }
    .
    .
}

Class Product.cs

public class Product
{
    /* PK */
    [Key]
    public int productId{ get; set; }
    public string name { get; set; }
    public decimal price { get; set; }
    public DateTime  purchase_date { get; set; }

    [Column("customer_id")]
    [ForeignKey("Customer")]
    public int? customer_id{ get; set; }
    public virtual Customer Customer { get; set; }
}

I appreciate any comments. I'm starting with language. If I need to, I can send some more codes.